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Abstract:

The present invention provides a method for producing silver
nanoparticles by employing ethanolamine. The method of this invention can
be easily operated and no organic solvent is required. Ethanolamine first
reacts with copolymers of poly(styrene-co-maleic anhydride) (abbreviated
as SMA) to generate polymeric polymers. The polymeric polymers then
reduce silver ions to silver atoms which are dispersed in the form of
silver nanoparticles. Functional groups of the polymeric polymers can
chelate with silver ions and be stably compatible with water or organic
solvents, whereby the silver nanoparticles can be stably dispersed
without aggregation and the produced silver nanoparticles.

Claims:

1. A method for preparing silver nanoparticles employing ethanolamine,
comprising the steps of: (A) reacting ethanolamine and
poly(styrene-co-maleic anhydride) copolymers (SMA) at 20.degree. C. to
30.degree. C. for 3 to 6 hours to generate a polymeric polymer, wherein
the ethanolamine has a general formula
(HOCH2CH2)3-zN(R)z, and wherein z=0, 1, or 2, and
R═H, alkyl, or alkenyl of C1 to C18; and (B) reacting the polymeric
polymers with silver ions at 50.degree. C. to 100.degree. C. for 5 to 24
hours to reduce the silver ions into silver atoms and disperse the silver
atoms as silver nanoparticles (AgNp).

2. The method of claim 1, wherein R in the ethanolamine is methyl, ethyl,
or cyclohexyl.

4. The method of claim 1, wherein the molar ratio of SMA to the amine
group of ethanolamine ranges from 1/10 to 2/1.

5. The method of claim 1, wherein the silver ions are provided from
AgNO3, and the weight ratio of polymeric polymers/AgNO3 ranges
from 1/99 to 99/1.

6. The method of claim 1, wherein step (B) is performed in an oil bath at
70.degree. C. to 90.degree. C.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a division of prior U.S. application
Ser. No. 12/836,762 filed Jul. 15, 2010, entitled "METHOD FOR PREPARING
SILVER NANOPARTICLES BY EMPLOYING ETHANOLAMINE". The prior U.S.
Application claims priority of Taiwan Patent Application No. 098124156,
filed on Jul. 16, 2009, the entirety of which is incorporated herein by
reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for preparing silver
nanoparticles, and particularly to a method for preparing silver
nanoparticles employing ethanolamine.

[0004] 2. Related Prior Arts

[0005] So far, methods for producing silver nanoparticles are classified
into physical methods and chemical methods. The physical method usually
demands expensive equipment for highly-vacuum vaporization or e-beam. The
chemical method uses reducers to reduce the silver ions to atoms and then
a stabilizer is used to control the size of the particles. Representative
reducers include NaBH4, formaldehyde, alcohol, hydrazine
(H2N--NH2) and the like. Representative stabilizers include
sodium citrate, glucose, sodium dodecyl sulfate, polyvinyl pyrrolidone
(PVP), dendrimer, and the like.

[0006] To avoid aggregation and promote the stability of the silver
nanoparticles, dispersants or protectors are usually added based on their
static electricity or steric hindrance. The dispersants can be water
soluble polymers, for example, polyvinylpyrrolidone (PVP),
polyvinylalcohol (PVA), polymethylvinylether, poly(acrylic acid) (PAA),
nonionic surfactants, chelating agents, etc.

[0007] Some stabilizers known in the art are disclosed in reports. In J.
Phys. Chem. B 1998, 102, 10663-10666, sodium polyacrylateor
polyacrylamide was provided as a stabilizer. In Chem. Mater. 2005, 17,
4630-4635, thioalkylated poly(ethylene glycol) was provided as a
stabilizer. In J. Phys. Chem. B 1999, 103, 9533-9539, sodium citrate was
provided as a stabilizer. In Langmuir 1996, 12, 3585-3589, nonionic
surfactants were provided as stabilizers. In Langmuir 1997, 13,
1481-1485, NaBH4 was provided as a reducing agent and anionic, cationic,
and nonionic surfactant were provided as stabilizers. In Langmuir 1999,
15, 948-951, 3-aminopropyltrimethoxysilane (APS) was provided as a
stabilizer and N,N-dimethyl-formamide was used as a reducing agent.

[0008] As described above, the traditional method for stabilizing silver
particles is to add surfactants or stabilizers. However, the solutions of
such silver particles have solid contents less than 10% and have a higher
solid content with aggregation.

[0009] Conventional chemical methods require the use of organic solvents,
salts, or reducing agents for long-term and complex redox reactions,
which result in high cost. Moreover, concentrations of the silver ions
have to be lowered to ppm scale during operation or the silver particles
will aggregate and perform undesired effects. Accordingly, there remains
a need for developing more efficient and cost effective methods for
preparing silver nanoparticles.

SUMMARY OF THE INVENTION

[0010] The object of the present invention is to provide a method for
preparing silver nanoparticles employing ethanolamine, which is simpler
than the conventional processes and does not require organic solvents.
Additionally, the generated silver particles can be uniformly and stably
dispersed at nanoscale without aggregation in high concentrations.

[0011] In the present invention, ethanolamine reacts with a mixture of
(poly(oxyalkylene)-amine)/epoxy or poly(styrene-co-maleic anhydride)
copolymers (SMA) to generate polymeric polymers, which further react with
silver ions to reduce the silver ions to silver and disperse the silver
as silver nanoparticles. Ethanolamine has a general formula:
(HOCH2CH2)3-zN(R)z, wherein z=0, 1, or 2, and R=H,
alkyl, or alkenyl of C1 to C18, such as methyl, ethyl, or cyclohexyl.
Examples of ethanolamine include monoethanolamine, diethanolamine,
triethanolamine, (±)-1-Amino-2-propanol (MPA), diglycolamine (DGA),
and N1,N1-dimethyl1-1,3-propanediamine (DAP).

[0012] In the reaction of ethanolamine and poly(oxyalkylene)-amine/epoxy,
the reaction temperature ranges from approximately 100° C. to
150° C. (preferably from 110° C. to 130° C.), and
the reaction time is about 5 to 10 hours. In the reaction of polymeric
polymers and silver ions, the reaction temperature ranges from about
15° C. to 35° C. (preferably from 20° C. to
30° C.), and the reaction time is about 12 to 36 hours.
Poly(oxyalkylene)-amine can be poly(oxyalkylene)-monoamine,
poly(oxyalkylene)-diamine, or poly(oxyalkylene)-triamine. Epoxy is
preferably diepoxides, for example, diglycidyl ether of Bisphenol-A or
3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate.

[0013] In the above reactions, the molar ratio of epoxy to the amine group
of ethanolamine preferably ranges from 1/3 to 3/1. The molar ratio of the
amine group of poly(oxyalkylene)-amine to the amine group of ethanolamine
preferably ranges from 1/5 to 5. The silver ions can be provided from
AgNO3, and the weight ratio of polymeric polymers/AgNO3
preferably ranges from 1/99 to 99/1.

[0014] In the reaction of ethanolamine and SMA/epoxy, the reaction
temperature ranges from about 20° C. to 30° C., and the
reaction time is about 3 to 6 hours. In the reaction of polymeric
polymers and silver ions, the reaction temperature ranges from about
50° C. to 100° C. (preferably in an oil bath from
70° C. to 90° C.), and the reaction time is about 5 to 24
hours.

[0015] In the above reactions, when the molar ratio of SMA to the amine
group of ethanolamine preferably ranges from 1/10 to 2/1 and the silver
ions is provided from AgNO3, the weight ratio of polymeric polymers
/AgNO3 preferably ranges from 1/99 to 99/1.

[0020] The method of the present invention primarily includes two steps:
(A) reacting ethanolamine and a mixture of poly(oxyalkylene)-amine/epoxy
or SMA to synthesize polymeric polymers; and (B) reducing silver ions
with the polymeric polymers to generate silver nanoparticles.

[0021] Ethanolamine of the present invention has a general formula:
(HOCH2CH2)3-zN(R)z, wherein z=0, 1, or 2, and R═H,
alkyl, or alkenyl of C1 to C18, such as methyl, ethyl or cyclohexyl.
Examples and structural formula of ethanolamine are shown in ATTACHMENT
1.

[0022] Epoxy has the following structural formula:

##STR00001##

[0023] The preferred examples of epoxy are shown in ATTACHMENT 2. Examples
of SMA are as follows:

[0024] Poly(oxyalkylene)-amine includes poly(oxyalkylene)-diamine,
poly(oxyalkylene)-monoamine, and poly(oxyalkylene)-amine having several
poly(oxyethylene) segments, which can be purchased from Huntsman Chemical
Co. or Aldrich Chemical Co.

[0025] Poly(oxyethylene)-monoamine has a general formula of R--NH2,
and the structural formula is:

##STR00003##

wherein a=0 to 10, and b=10 to 50.

[0026] For example, Jeffamine® M-2070 has a molecular weight of
approximately 2000, and a=10 and b=31 in the above formula.

[0027] Poly(oxyethylene)-diamine has a general formula of
H2N--R--NH2, and the structural formula is:

##STR00004##

wherein a=10 to 50, and b or c=0 to 10.

[0028] For example, Jeffamine® ED-2003 has a molecular weight of
approximately 2000, includes oxyethylene (EO) and oxypropylene (PO)
segments, and a+c=6 and b=39 in the above formula.

[0029] Other examples of poly(oxyalkylene)-amine are shown in ATTACHMENT
3.

[0030] In the following detailed description, the silver ions were
provided from AgNO3 (99.8 wt %) purchased from Aldrich Co. However,
other silver salts such as AgI, AgBr, AgCl, and silver
pentafluoropropionate are also suitable.

[0031] Detailed procedures are described as follows:

EXAMPLE 1

Step (A): Synthesizing the Polymeric Polymer BE188/ED2001/MEA

[0032] ED2001 was dewatered in vacuum at 120° C. for 6 hours. In a
500 ml three-necked bottle, diglycidyl ether of bisphenol A (BE188) (7 g,
0.02 mol), ED2001 (40 g, 0.02 mol) and MEA (1.22 g, 0.02 mol) were added
so that the molar ratio of BE188/ED2001/MEA was 1/1/1. The mixture was
mechanically mixed and reacted in nitrogen at 120° C. for more
than 5 hours. The mixture was sampled at intervals for IR analysis until
the characteristic peak of the epoxy group disappeared on FT-IR spectrum.
After the reaction completed, the product, a light yellow viscous liquid,
was observed. FIG. 1 shows the reaction.

Step (B): Synthesizing Silver Nanoparticles

[0033] BE188/ED2001/MEA (0.2 g) was dissolved in water (10 g) in a
three-necked bottle. AgNO3 (0.05 g) was mixed and reacted at room
temperature for one day and the solution became black. The UV analysis
showed that the silver nanoparticles were generated according to
characteristic absorption thereof at wavelength 430 nm.

EXAMPLES 2 to 3

[0034] Repeat procedures of Example 1, except that the molar ratio of
BE188/ED2003/MEA was changed to 2/1/2 and 3/1/3, respectively. The silver
nanoparticles having good thermal stability in a high concentration were
prepared.

EXAMPLE 4

[0035] Repeat procedures of Example 1, except that MEA was changed to DEA.
The silver nanoparticles having good thermal stability in a high
concentration were prepared.

EXAMPLES 5 to 6

[0036] Repeat procedures of Example 4, except that the molar ratio of
BE188/ED2003/DEA was changed to 2/1/2 and 3/1/3, respectively. The silver
nanoparticles having good thermal stability in a high concentration were
prepared.

EXAMPLES 7 to 8

[0037] Repeat procedures of Example 1, except that MEA was changed to DGA
and DAP, respectively. The silver nanoparticles having good thermal
stability in a high concentration were prepared.

EXAMPLE 9

Step (A): Synthesizing the Polymeric Polymer SMA/MEA

[0038] SMA and MEA were dewatered in vacuum at 120° C. for 6 hours
and subsequently dissolved in tetrahydrofurane (THF). Next, MEA (5.2 g,
85.6 mmol) was placed in a three-necked bottle, and SMA1000 (10.0 g,
including 42.8 mmol MA, dissolved in 50 mL THF) was added therein by
several batches to avoid cross-linking. The reaction time was 3 to 6
hours. The synthesized polymer SMA/MEA was insoluble in THF. By vacuum
filtration, the polymer was separated from THF and excess MEA. The
reaction is shown in FIG. 2. SMA/MEA was then dissolved in different
solvents and the result showed the best compatibility in water, ethanol
as the next, and insolubility in toluene, methyl ethyl ketone (MEK),
acetone, and isopropyl alcohol (IPA).

Step (B): Synthesizing Silver Nanoparticles

[0039] In a round-bottom flask, SMA/MEA (0.015 g) was dissolved in water
(50 g) and stirred with a magnetic stirrer. AgNO3 (0.005 g) was then
added for preparing silver nanoparticles through a reductive reaction in
an oil bath at 80° C. for 5 hours. With increasing concentration
of the silver nanoparticles, the solution became brown from light yellow.
The UV analysis showed that the silver nanoparticles were generated
according to characteristic absorption thereof at wavelength 425 nm.

EXAMPLES 10 to 12

[0040] Repeat procedures of Example 9, except that the weight ratio of
AgNO3 to dispersant SMA/MEA of step (B) was changed as 1/5, 1/7 and
1/9, respectively. With UV analysis, the relationship of the amounts of
the dispersants to reaction time is shown FIG. 3 in which the weight
ratio of AgNO3 to the dispersant SMA/MEA did not changed with UV
absorption. That is, the reductive reaction completed after 5 hours. If
the amounts of the dispersant increased, the reaction time decreased.

EXAMPLES 13 to 14

[0041] Repeat procedures of Example 9, except that MEA was changed as DEA
and MPA, respectively. The silver nanoparticles having good thermal
stability in a high concentration were prepared.

Comparative Example 1

Step (A): Synthesizing PMDA/ED2001/MEA

[0042] In a 100 ml three-necked bottle, ED2001 (10 g, 0.005 mol) was added
and dissolved in THF (10 ml). PMDA (2.18 g, 0.01 mol) was then added so
that the molar ratio of PMDA/ED2003/MEA was 2/1/2. By mechanically
blending, the reaction was performed in nitrogen below 30° C. for
at least 2 hours. The mixture was sampled at intervals for IR analysis
until the characteristic peak of the amide group disappeared on FT-IR
spectrum. After the reaction completed, MEA (0.61 g, 0.01 mol) was added
and peak of the anhydride functional group disappeared.

[0044] Repeat step (B) of Example 1, except that BE188/ED2001/MEA was
replaced with PMDA/ED2003/MEA. As a result, the silver ions were stable
but could not be reduced into silver nanoparticles unless strong reducing
agents such as NaBH4, was added. Thus, the stablizers synthesized
according to the present invention were necessary.

Comparative Example 2

[0045] Repeat procedures of Example 1, except that BE188/MEA was
synthesized in step (A) and replaced BE188/ED2003/MEA in step (B).
Finally, the dispersant was not soluble in water.

Comparative Example 3

[0046] Repeat procedures of Example 1, except that BE188/ED2003 was
synthesized in step (A) and replaced BE188/ED2001/MEA in step (B).
Finally, a significant amount of silver particles settle down on the
bottom of the bottle. Thus, the stablizers synthesized according to the
present invention were necessary.

[0048] Operation conditions of the above Examples and Comparative Examples
were listed in ATTACHMENT 4.

Preparing the Concentrative Dispersions of the Silver Nanoparticles

[0049] After being stabilized with polymeric polyamines of the present
invention, the silver nanoparticles could be further concentrated by a
water-jet concentrator or a freezing dryer to achieve silver paste,
silver gel, or silver powders having concentrations at least 10 wt %,
even more than 30 wt %.

[0050] According to the above description, features of the present
invention are summarized as follows:

1. The polymeric polymers can act as both a reducing agent and a
stabilizer (or dispersant) in preparing the silver nanoparticles because
functional groups thereof, for example, carboxylic acid, multi-amine,
amide, and hydroxyl group, can chelate with silver ions. 2. The molar
ratios of polymeric polymers (dispersant) to silver particles can be
controlled to limit the silver particles at nanoscale, generally about
100 nm, and even smaller than 10 nm. 3. The silver nanoparticles can be
uniformly and stably dispersed in much higher concentrations than the
commercial silver products and can be further concentrated to form a
silver paste which can be dispersed in a medium again. The medium can be
a hydrophilic solvent such as water or a hydrophobic organic solvent such
as methanol, ethanol, IPA, acetone, THF, MEK, toluene, and the like. 4.
The silver nanoparticles can be blended in organic polymers at nanoscale
to form composites of good electrical conductivity or germproof effects.
The organic polymers can be polyimide (PI), epoxy, nylon, polypropylene
(PP), acrylonitrile butadiene styrene (ABS), polystyrene (PS), and the
like.

Patent applications by Chao-Po Hsu, Taipei TW

Patent applications by Jiang-Jen Lin, Taipei TW

Patent applications by Wei-Cheng Tsai, Taipei TW

Patent applications by Wei-Li Lin, Taipei TW

Patent applications by Yueh-Hsien Wu, Taipei TW

Patent applications by NATIONAL TAIWAN UNIVERSITY

Patent applications in class Using nonmetallic material which is liquid under standard conditions

Patent applications in all subclasses Using nonmetallic material which is liquid under standard conditions